DUBAI, July 1 (Reuters) - The leader of the al Qaeda offshoot now calling itself the Islamic State has called on Muslims worldwide to take up arms and flock to the 'caliphate' it has declared on captured Syrian and Iraqi soil.

Proclaiming a "new era" in which Muslims will ultimately triumph, Abu Bakr al-Baghdadi issued the call to jihad - holy war - in an audio message lasting nearly 20 minutes that was posted online on Tuesday.

It was his first purported message since the group - previously known as the Islamic State in Iraq and the Levant (ISIL) - proclaimed the caliphate on Sunday and declared him its leader, in an audacious bid to sweep away state borders and redraw the map of the Middle East.

Baghdadi, who has assumed the mediaeval title of caliph, used the message to seek to assert authority over Muslims everywhere. He called on them to rise up and avenge the alleged wrongs committed against their religion, from Central African Republic to Myanmar (Burma).

"Terrify the enemies of Allah and seek death in the places where you expect to find it," he said. "Your brothers, on every piece of this earth, are waiting for you to rescue them."

The audio message, titled "A Message to the Mujahideen and the Muslim Ummah in the Month of Ramadan," was posted online through the group's media arm. Another account affiliated to the group posted translations in English, Russian, French, German and Albanian.

"By Allah, we will take revenge, by Allah we will take revenge, even if after a while," Baghdadi said.

"THUNDERING STATEMENT"

The audio message's authenticity could not be immediately verified, but it was carried by SITE, an authoritative U.S.-based organisation that monitors jihadist statements.

Fighters should "embrace the chance and champion Allah's religion through jihad", Baghdadi said.

He called on Muslims to immigrate to the self-styled caliphate, saying it was their duty. In a direct, confident message, he urged them to "listen, realise and stand and free yourself from the shackles of weakness, and stand in the face of tyranny".

"Let the world know that we are living today in a new era. Whoever was heedless must now be alert. Whoever was sleeping must now awaken. Whoever was shocked and amazed must comprehend. The Muslims today have a loud, thundering statement, and possess heavy boots," said Baghdadi, according to the posted translation.

"They have a statement that will cause the world to hear and understand the meaning of terrorism, and boots that will trample the idol of nationalism, destroy the idol of democracy and uncover its deviant nature."

"VANGUARD OF NEW ISLAMIC AWAKENING"

The declaration of the caliphate followed a three-week drive for territory by ISIL militants and their allies among Iraqi's Sunni Muslim minority.

Dominating swathes of territory in an arc from Aleppo in Syria to near the western edge of Baghdad, the caliphate aims to erase colonial-era borders and defy the U.S.- and Iranian-backed government of Iraq's Shi'ite prime minister, Nuri al-Maliki.

It also poses a direct challenge to the global leadership of al Qaeda, which has disowned ISIL, and to conservative Gulf Arab Sunni rulers, who already view the group as a security threat.

Julian Barnes-Dacey, a senior policy fellow with the European Council on Foreign Relations, said Baghdadi had made a "bold call in proclaiming this caliphate and speaking out so vigorously now".

"He perceives this as his moment, having been able to seize this unprecedented amount of territory. It's a bold, all-in strategy wherein he is trying to present himself as the vanguard of this new Islamic awakening," Barnes-Dacey said.

Earlier on Tuesday, Shi'ites failed to name a prime minister to replace Maliki at the first meeting of a new parliament session, dashing hopes that a unity government would be swiftly built to save Iraq from collapse.

While ISIL's power grab may have appeal to many, there have already been signs of dissent. Some Islamist groups fighting in Syria have rejected the announcement of the caliphate, saying its terms had not "been realised at present", and urged Muslims to avoid siding with the Islamic State.

"So even as he (Baghdadi) made a bold proclamation of leadership, there remains a great deal of uncertainty about the impact it's going to have and the legitimacy that it's going to give him to mobilise a constituency," Barnes-Dacey said. (Additional reporting by Ali Abdelaty in Cairo, Amena Bakr in Doha and Dominic Evans in Beirut; Writing by Yara Bayoumy; Editing by Mark Trevelyan)

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